The present invention relates to an RF power amplifier and an RF power amplifier apparatus for RF transmission, each of which is mounted in a communication terminal device like a cellular phone terminal that communicates with a base station. Further, the present invention particularly relates to a technique beneficial to realize two functions of a non-saturation type linear amplifier and a saturation type nonlinear amplifier by one RF power amplifier.
The operation of a high-frequency power amplifier included in a cellular phone terminal has a saturation operation in GSM of a basic mode in which only phase modulation is used. EDGE that uses amplitude modulation along with phase modulation has linear operation at an operating point where several dB are backed off from a saturation operating point of GSM. Even in the case of WCDMA and cdma-1x that also use amplitude modulation along with phase modulation, the operation of the high-frequency power amplifier has linear operation.
At a high-frequency circuit portion of the cellular phone terminal corresponding to each of GSM and EDGE, an antenna switch is disposed between the high-frequency power amplifier and an antenna. The antenna switch performs the function of switching between transmission and reception slots of a TDMA (Time Division Multiple Access) system.
At a high-frequency circuit portion of a cellular phone terminal corresponding to each of WCDMA and cdma-1x, a duplexer is disposed between a high-frequency power amplifier and its corresponding antenna. The duplexer performs the function of processing in parallel, transmission of an RF transmit signal of a low RF frequency of a CDMA (Code-Division Multiple Access) system and reception of an RF receive signal of a high RF frequency thereof. Further, in WCDMA and cdma-1x or the like, an isolator is disposed between the high-frequency power amplifier and the duplexer to avoid the influence of a variation in load at the antenna over the high-frequency power amplifier. Since it is however difficult for the isolator to be integrated into a structure at which the high-frequency power amplifier is manufactured, it becomes a large and expensive part.
Ubiquitous coverage corresponding to the capability of a communication terminal device such as a cellular phone terminal that wireless communications are carried out in any place in the world is not real in these days, but now proceeding into development.
According to a non-patent document 1 (Earl McCune, “High-Efficiency, Multi-Mode, Multi-Band Terminal Power Amplifiers”, IEEE microwave magazine, March 2005, PP. 44-55), these mobile systems respectively include cells of GSM, GPRS, EDGE and WCDMA, and networks of, for example, IEEE 802.11-b, -a and -g or the like, such as personal area networks such as Bluetooth and ZigBee. The characteristics of these systems extend to signals for a constant envelope and a change in envelope, multiplex time division and code division and a wider combination of transmit output power from high (few watts) to low (micro watts). As a result, there has been a growing demand for a multimode-applied RF power amplifier.
A self-evident approach to the multimode is to apply a linear circuit technique in order to support an envelope change signal. This approach however causes a basic contradiction in the circuit design of a power amplifier. As is well known, the maximum efficiency of the power amplifier is obtained by allowing the power amplifier to perform a saturation operation (nonlinear operation at which waveform clips occur). Since the power amplifier that performs the saturation operation operates as a limiter where an input signal is of an envelope change signal, serious signal distortion occurs. Thus, the power amplifier needs to perform a non-saturation linear operation in order to reproduce the envelope change signal faithfully. To this end, the concept of output back-off is introduced and the power amplifier is limited to the linear operation at which the peak output power of the power amplifier that performs the non-saturation linear operation is lower than the maximum (saturated) output power. Since, however, the output back-off becomes difficult in design, two discrete power amplifiers corresponding to a non-saturation type linear amplifier and a saturation type nonlinear amplifier have been developed.
Incidentally, GSM is an abbreviation of Global System for Mobile Communication. GPRS is an abbreviation of General Packet Radio Service. Further, EDGE is an abbreviation of Enhanced Data for GSM Evolution (Enhanced Data for GPRS). WCDMA is an abbreviation of Wideband Code Division Multiple Access.
On the other hand, an RF power amplifier module that transmits a quadband including frequency bands of GSM850, GSM900, DCS1800 and PCS1900 has been described in non-patent document 2 (Shuyun Zhang et al, “A Novel Power-Amplifier Module for Quad-Band Wireless Handset Applications”, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 51, No. 11, NOVEMBER 2003, PP. 2203-2210). Incidentally, DCS is an abbreviation of a Digital Cellular System and PCS is an abbreviation of a Personal Communication System. The RF power amplifier module includes a first power amplifier which amplifies a first RF transmit input signal having a first frequency band of GSM850 and GSM900, and a second power amplifier which amplifies a second RF transmit input signal having a second frequency band of DCS1800 and PCS1900.
In communications of GSM850, GSM900, DCS1800 and PCS1900, a TDMA system has been adopted which is capable of setting a plurality of time slots to any of an idle state, a reception operation from a base station and a transmission operation to the base station, respectively, on a time-sharing basis. Incidentally, TDMA is an abbreviation of Time-Division Multiple Access. As one TDMA system, there is known a GSM system that uses only phase modulation. There is also known a system that improves a communication data transfer rate as compared with the GSM system. As an improvement system, an EDGE system that uses amplitude modulation along with the phase modulation has recently been brought to attention.
On the other hand, a WCDMA system that has improved a communication data transfer rate by using amplitude modulation along with phase modulation has also been a focus of attention in recent years in a manner similar to the EDGE system. In this WCDMA system, there has been adopted a frequency-division CDMA system other than the TDMA system, which uses frequencies of 2110 MHz to 2170 MHz for a reception operation from a base station and uses frequencies of 1920 MHz to 1980 MHz for a transmission operation to the base station. Incidentally, CDMA is an abbreviation of Code Division Multiple Access.
A non-patent document 3 (Gary Hau et al, “High Efficiency, Wide Dynamic Range Variable Gain and Power Amplifier MMICs for Wide-Band CDMA Handsets”, IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 11, No. 1, JANUARY 2001, PP. 13-15) has described that since a wider control range and high linearity are necessary for power control of an RF power amplifier of the WCDMA system, a programmable gain amplifier using a variable attenuator is coupled to the input of the RF power amplifier.
On the other hand, power control of an RF power amplifier by a closed loop and source voltage control has been described in a non-patent document 4 (Angelo Scuderi et al, “A VSWR-Protected Silicon Bipolar RF Power Amplifier With Soft-Slop Power Control”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 40, No. 3, MARCH 2005, PP. 611-621). In the power control by the closed loop, RF power output of the amplifier is sensed using a directional coupler and detected by a diode. A detected voltage is compared with a reference voltage by an error amplifier. The output of the error amplifier drives a gain control terminal of the power amplifier to equally control the detected voltage and the reference voltage by the closed loop. The power control is realized by a change in the reference voltage. In the source voltage control of the RF power amplifier, which controls the output power thereof, a linear regulator configured by a power PMOS transistor and an operational amplifier is used, and a collector voltage of the RF power amplifier is linearly changed by a control terminal of the operational amplifier. The output amplitude at which power is obtained is limited by reducing the collector voltage.
Closed-loop collector peak voltage control for coping with a breakdown based on a high voltage peak at the collector of a final stage in a state of a high-load voltage standing wave ratio (VSWR) caused by load mismatching has been described in the non-patent document 4. This control is configured by an AC sense circuit/envelope detector that detect a peak voltage at an output collector node, and an error amplifier which changes circuit gain thereby to clamp the peak voltage to a specific threshold voltage. The error amplifier controls a bias current of a drive stage for driving the final stage.
A parallel power amplifier that realizes low distortion and high efficiency at a wide range of load impedance without using an isolator has been described in a non-patent document 5 (Hikaru Ikeda et al, “A Low Distortion and High Efficiency Parallel-Operation Power Amplifier Combined in Different Phases in Wide Range of Load Impedance”, 1996 IEEE MTT-S Digest, pp. 535-538). The parallel power amplifier has a plurality of amplification paths. A signal inputted to one input terminal is supplied to the inputs of the amplification paths by a hybrid divider. Each of the amplification paths includes an amplifier and a phase shifter. The phase shifters are disposed on their corresponding amplification paths in such a manner that the phases of operations of the amplifiers differ from one another at the amplification paths. Plural outputs of the amplification paths are coupled to a single output by a hybrid coupler. The non-patent document 5 has described that at a 3:1 VSWR (Voltage Standing-Wave Ratio) or less in which reflectivity Γ is equivalent to 0.5, a distortion of −45 dBc or less, an efficiency of 45% or more and a gain of 9.8 dB or more have been obtained. Incidentally, VSWR is an abbreviation of Voltage Standing-Wave Ratio. VSWR is determined by VSWR=(1+Γ)/(1−Γ) in accordance with reflectivity Γ.
A balanced amplifier similar to the parallel power amplifier described in the non-patent document 5 has been described in a non-patent document 6 (Giuseppe Berrtta et al, “A balanced CDMA SiGe HBT Load Insensitive Power Amplifier”, 2006 IEEE Radio and Wireless Symposium 17-19 Jan. 2006, PP. 523-526) to adapt to the mismatching of a load due to the omission of the isolator. The balanced amplifier includes an input hybrid coupler, two RF power amplifiers, two matching circuits, an output hybrid coupler and two 50Ω terminal resistors. The supply of an RF input signal to input-side two terminals of the input hybrid coupler and the coupling of the 50Ω input terminal resistors thereto are carried out. Two input terminals of the two RF power amplifiers are coupled to their corresponding two output-side terminals of the input hybrid coupler. Two input terminals of the two matching circuits are coupled to their corresponding two output terminals of the two RF power amplifiers. Two input-side terminals of the output hybrid coupler are coupled to their corresponding two output terminals of the two matching circuits. The coupling of the 50Ω output terminal resistors to their corresponding two output-side terminals of the output hybrid coupler and the coupling of an antenna thereto are carried out. The parallel power amplifier demonstrates a satisfactory input/output return loss and has insensitivity satisfactory for variations in a load.